GOP leaders announced an effort to invalidate many of California's new voting districts Monday as the boundaries were finalized by the commission that drew them.

The maps, drafted for the first time by a citizens' panel rather than politicians, could give Democrats a tighter grip on the statehouse and California's congressional delegation. In particular, the new lines put Democrats within reach of the coveted two-thirds majority, which is needed to raise taxes, in the state Senate.

The California Citizens Redistricting Commission consists of five Democrats, five Republicans and four unaffiliated members. But state GOP Chairman Tom Del Beccaro characterized the approved boundaries as "unfair if not unconstitutional."

Republicans aren't the only ones girding to fight the new maps, which are to be used during the next decade in elections for 120 seats in the state Legislature, 53 in Congress and four on the state Board of Equalization. Activists argue that Latinos are underrepresented in some new districts and are threatening a court challenge.

Even some on the 14-member commission expressed reservations about its final product, emboldening potential challengers.

A referendum drive to overturn the state Senate lines is being led by state Sen. Mimi Walters (R-Laguna Niguel) and Orange County businesswoman Julie Vandermost through a committee called Fairness and Accountability in Redistricting, according to Republican consultant Dave Gilliard. They may also launch a referendum on the congressional boundaries, according to Gilliard.

"It is our goal, and should be the goal of all elected officials who believe in fair, accountable and transparent government, to reject the lines drawn and ensure that the referendum is successful," Walters said.

Backers have 90 days to collect 504,000 signatures to qualify a referendum for the ballot.

The petition drive is endorsed by the 15-member Senate Republican Caucus, according to state Senate minority Leader Bob Dutton of Rancho Cucamonga.

"I indicated my willingness to try to help raise some money for the referendum," Dutton said. "The people of California were hoping for an open and free process that was free of political influence and I'm not so sure that's what they got."

If a referendum makes it to the ballot, the redistricting plan adopted Monday will be suspended and the state Supreme Court will determine districts for the 2012 election. GOP strategists say that's an easy bet.

"The idea that the court would disregard the 500,000-plus citizens' signatures to leave in place a plan approved by 14 citizens — no one I know thinks that's a likely outcome," said Jim Brulte, a Republican and former state legislator.

Bruce Cain, a UC Berkeley political scientist and reapportionment expert, said that even if a referendum qualifies, it is unlikely to pass. "For the most part," he said, "public opinion has been relatively positive."

The president of the Mexican American Legal Defense and Educational Fund, meanwhile, said his organization is considering taking the commission to court.

"We are looking at compliance with the federal Voting Rights Act," said MALDEF President and General Counsel Thomas A. Saenz, citing particular concerns over the state Senate and congressional district lines. The federal law protects minorities against having their voting strength diluted.

Saenz suggested the commission failed to create enough new districts with Latinos as the majority of voters. That raises a potential legal issue, he said.

The state Supreme Court on Monday announced an expedited process for hearing legal challenges to the new maps. But Saenz said his group may file a challenge in federal court, where the case would be heard by a three-judge panel that could consider requests for a preliminary injunction blocking the Senate districts from taking effect.

One of the redistricting commissioners, Maria Blanco, expressed reservations similar to MALDEF's before casting her vote to approve the plan. She noted that the state Senate district now represented by Democrat Alex Padilla of Pacoima in the San Fernando Valley was changed into one with far fewer Latinos of voting age.

Commissioner Michael Ward voted against all of the new legislative maps, the only member of the panel to do so. He accused his colleagues of violating their legal mandate by making some decisions based on "political" considerations.

"This commission broke the law," Ward, a Republican chiropractor from Anaheim, said at a Capitol news conference attended by all 13 other members of the panel. He said they held secret meetings to draw boundaries for partisan considerations.

"This commission simply traded the partisan, backroom gerrymandering by the Legislature for partisan, backroom gerrymandering by average citizens," Ward said.

Ward's charge was disputed by commission Chairman Vincent Barabba, a Republican businessman from Santa Cruz County. He said decisions were made in open meetings, and there was "no basis" for the accusation that they were due to political considerations.

"The sense I get is that Commissioner Ward attended different meetings than I did," Barabba said.